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CC115L: Frequency shift with some devices

Part Number: CC115L

Have product transmitting on 900 Mhz band with the CC115L. However, some devices are at a -40Khz shift while others are right on.  After replace the CC115L with another one, the shift is gone.   Note: There is no communication as the -40Khz seems to put it out of band.

The spec states a 40PPM. Calculating here: 40KHz/900Mhz= 44 PPM which shows out of spec.

Looks like if I reduce the over all capacitance of the bypass caps (30pf each) a bit, it should bring it back into band.  But, can't good too much as it would too high on devices that don't have the lower shift.

Question is why some device seem to add to the shift while others do not?  Can I assume that the circuit is on the edge of the spec and the tolerance of the device then comes into play?  Keep in mind, the shift is either 40Khz or not and nothing in between.

Thanks  Jah.

  • A frequency shift is set by the reference. In other words, if the reference is 10 ppm off you will also see 10 ppm off on the air. The reference is a xtal on CC115L (I assume).

    Could you confirm that you measure the frequency sending a CW (tone) and using a small RBW (< 3 kHz) on the spectrum analyzer to measure.

    Could you also confirm that you have run a SCAL before the measurement.

    I have not seen the frequency varies > 40 ppm sample to sample. Which xtal do you use and which ppm spec does this have? Do you use a Cload that is within spec for both the chip and xtal?
  • To answer your questions:


    Scal is being used.

    Xtal is: SM 26.000 mHZ Q26.0-XS53-18-10/10-FU-LF by Jauch.

    We also have FW with CW on them but you can load other FW if you like.

    The issue follow the 115 device; If you exchange the RF chips from the good to the bad and vice versa the change is going with the RF chip. The bad PCBA is becoming good and the good becomes bad.


  • This is a very surprising result. So if I read this correctly, you had a PCB with 40 ppm frequency offset (board 1) and one with 0 ppm frequency offset (board 2) and if you move the RF chip from board 1 to board 2 (running the same software) board 2 has a frequency offset equal to 40 ppm?